Perpendicular magnetic recording (PMR) techniques may enable higher recording densities on magnetic storage media than conventional longitudinal magnetic recording techniques. PMR systems include heads that record bits perpendicular to the plane of the disk. PMR disks include a high permeability (“soft”) magnetic underlayer between a perpendicularly magnetized thin film data storage layer and the substrate. An image of the magnetic head pole created by the head is produced in the magnetically soft underlayer. Consequently, the storage layer is effectively in the gap of the recording head, where the magnetic recording field is larger than the fringing field produced by a longitudinal magnetic recording (LMR) head. The larger recording field makes it possible to record using smaller grain sizes and smaller bit sizes than in LMR systems.
In PMR, the channel response has a DC component. For a channel that is AC-coupled to the preamplifier and read channel, or that contains some other means for high-pass filtering the channel response, there may be DC-distortion. The DC-distortion may manifest itself as a data dependent baseline wander, which can severely affect the performance of a system that equalizes the channel response to a response target that is not DC-free.